This invention relates to an image forming apparatus that reads an image from an original document and forms a developer image corresponding to the image read, on a paper sheet. The fixing apparatus provided in the image forming apparatus fixes the developer image formed on the paper sheet.
The fixing apparatus described above takes the paper sheet in the nip between a heat roller and a press roller and applies heat and pressure to the paper sheet, thereby fixing the developer image on the paper sheet. A center coil and side coils, each designed to perform induction heating, are provided on the inner surface of the heat roller or close to the outer surface thereof. A high-frequency current is supplied to these coils, which generate a high-frequency magnetic field. The magnetic field generates an eddy current in the heat roller. The eddy current brings forth Joule heat. The Joule heat heats the heat roller.
The center coil performs induction heating, heating a part of the heat roller, which is substantially middle in the axial direction of the heat roller. One of the side coils heats one end part of the heat roller. The other side coil heats the other end part of the heat roller. The center coil, on the one hand, and the side coils are driven, on the other, are alternately driven, each for a controlled time, so that the temperature T1 of the center part of the heat roller and the temperature T2 of the end parts thereof may have a preset value Ts, no matter whether a paper sheet exists or whatever size a paper sheet, if any, has (A4-R size, A3 size or the like). Alternatively, the center coil and the side coils may be driven at the same time to have their outputs controlled.
In a warming-up operation period immediately after the power switch of the image forming apparatus is closed, the center coil and the side coils are so driven that the temperatures T1 and T2 may quickly rise to the preset value Ts. The temperatures T1 and T2 rises at different rates, however, due to the difference between the heat capacity of the fixing apparatus and the design heat capacity that the fixing apparatus should have or due to the environmental changes that influence the fixing apparatus. For example, the temperature T2 of one end part of the heat roller rises to the preset value Ts some time after the temperature T1 of the middle part of the heat roller reached the preset value Ts. In this case, the warming-up operation is terminated when the temperature T2 reaches the preset value Ts. Consequently, the warming-up operation period becomes longer than is desired. Further, the heat emanating from the center coil for the time that is the difference between the desired warming-up operation period and the actual warming-up operation period is inevitably wasted.